Ohmic contact electrodes for N-type semiconductor cubic boron nitride

ABSTRACT

An ohmic contact electrode formed on an n-type semiconductor cubic boron nitride by using a IVa metal or an alloy with a IVa metal or a layer of Au or Ag.

This is a divisional of application Ser. No. 07/705,594, filed May 24,1991, now U.S. Pat. No. 5,285,109.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an ohmic contact electrode for ann-type semiconductor cubic boron nitride.

2. Description of Related Art

Attention has been given to a semiconductor cubic boron nitride as a newmaterial for semiconductor devices such as diodes, transistors, sensors,and so on. Although an insulating cubic boron nitride is known broadly,that which is discussed here is a semiconductor cubic boron nitridehaving low resistivity.

A cubic boron nitride has a wide forbidden band width (7.0 eV) and ahigh heat-proof temperature (1300° C.), and is chemically stable.Consequently, a semiconductor cubic boron nitride has been stronglyexpected to be an excellent material for environment-proof powerdevices, and blue light emission elements.

A cubic boron nitride does not naturally exist but can be formed throughhigh-pressure synthesis. Further, recently, the formation of thin filmthrough vapor phase synthesis has been reported.

A p-type cubic boron nitride can be obtained by doping beryllium (Be).

An n-type cubic boron nitride, on the other hand, can be obtained bydoping sulfur (S) or silicon (Si). At present, a pn-Junction diode isproduced for trial by using a high-pressure synthetic method in such amanner that an n-type cubic boron nitride is continuously grown with ap-type cubic boron nitride as a seed crystal. The pn-junction diodewhich is produced has a diode characteristic under 650° C. Further, ithas been reported that a pn-junction diode emits, by injection ofminority carries, light from ultraviolet to blue centering around 340nm.

A contact between a metal and a semiconductor is either a Schottkycontact or an ohmic contact. The Schottky contact has a rectificationproperty so that a current does not flow in a reverse direction. In thecase of producing semiconductor devices, it is very important to formelectrodes which can be formed by ohmic contact. The term "ohmic contactelectrode" means an electrode in which the characteristic of a currentflowing through the electrode and the characteristic of a voltage acrossthe electrode are symmetrical forward and backward in accordance withthe Ohm's law. Further, it is preferable that the contact resistance isreduced as much as possible. The term "contact resistance" is defined asa voltage to be applied so as to make a unit current flow through a unitcontact surface. The unit of the contact resistance is Ω.cm².

There has not yet been found a superior electrode for an n-type cubicboron nitride. Although silver (Ag), silver paste, or the like is usedas an electrode for an n-type cubic boron nitride ((1) R. H. Wentorf,Jr.: J. of Chem. Phys. Vol. 36 (1962) 1990; (2) O. Mishima, etc.:Science Vol. 238 (1987) 181), there has been no report that an ohmiccontact could be obtained.

The technique for forming ohmic contact is necessary and indispensableto produce semiconductor devices. Further, if an ohmic contact isobtained, it is desirable that the contact resistance in the ohmiccontact portion be reduced as much as possible. The contact resistanceof general electron devices is 10⁻² Ω.cm² or less, and a smaller contactresistance of 10⁻⁴ Ω.cm² or less is required in high-speed andhigh-frequency devices.

If a formed contact is not an ohmic one but a Schottky one, carrierscannot be effectively injected into a produced device because of theexistence of a Schottky barrier in the contact electrode portion evenwhen the current is to be made to flow in the device. This results inlow efficiency of carrier injection. Further, the voltage drop acrossthe contact electrode portion is remarkably large because the resistanceis larger at the contact electrode portion. Therefore, the effectivevoltage applied to the device ends up being small. As a result, theparticularly desireable characteristics discussed above for the devicecannot be obtained. Further, the generation of heat in the contactelectrode portion is a large problem.

Accordingly, the formation of an ohmic contact electrode is necessaryand indispensable in order to make it possible to utilize asemiconductor cubic boron nitride as a material for semiconductordevices.

An object of the present invention is therefore to form an ohmic contactelectrode on an n-type semiconductor cubic boron nitride.

SUMMARY OF THE INVENTION

The ohmic contact electrode for an n-type semiconductor cubic boronnitride according to the present invention is an electrode formed on ann-type semiconductor cubic boron nitride so that an ohmic contactelectrode can be obtained by using:

(1) a IVa-family metal which includes titanium (Ti), zirconium (Zr), andhafnium (Hf) or an alloy containing a IVa-family metal;

(2) a metal or alloy containing silicon (Si) or sulfur (S);

(3) a metal or alloy containing at least one of boron (B), aluminum(Al), gallium (Ga), and indium (In); or

(4) a Va-family metal which includes vanadium (V), niobium (Nb), andtantalum (Ta) or an alloy containing a Va-family metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a band diagram in the case where a high concentration dopedlayer is formed in the vicinity of a metal/semiconductor interface whenone of the following materials is applied to an n-type semiconductorcubic boron nitride:

(1) IVa-family metal or an alloy containing a IVa-family metal;

(2) a metal or alloy containing Si or S;

(3) a metal or alloy containing at least one of B, Al, Ga and In; or

(4) a Va-family metal or an alloy containing a Va-family metal.

FIG. 2 is a band diagram in the case where high-density localized statesare formed in the vicinity of a metal/semiconductor interface when oneof the following materials is applied to an n-type semiconductor cubicboron nitride:

(1) a IVa-family metal or an alloy containing a IVa-family metal;

(2) a metal or alloy containing at least one of B, Al, Ga and In; or

(3) a Va-family metal or an alloy containing a Va-family metal.

FIG. 3 is a plan showing an electrode pattern which was used inmeasuring the contact resistance between the semiconductor cubic boronnitride and the electrodes. In the drawing, hatched portions representthe electrodes:

1 . . . Fermi-level of electrode;

2 . . . Fermi-level of semiconductor;

3 . . . lower end of conduction band of semiconductor;

4 . . . upper end of valence band of semiconductor;

5 . . . electrode/semiconductor interface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, an ohmic contact electrode can beobtained by coating an n-type cubic boron nitride with the following:

(1) A IVa-family metal (Ti, Zr, or Hf) or an alloy containing aIVa-family metal;

(2) A metal or alloy containing Si or S which is contained in a matrixmaterial desirably selected from Cr, W, Mn, Co, Ni, Pt, Ag, Zn, WC,MnZn, Ni₂ B, PtPd, AgAl, and so on;

(3) A metal or alloy containing at least one of B, Al, Ga and In; or

(4) A Va-family metal (V, Nb, or Ta) or an alloy containing a Va-familymetal.

This technique has been found through repeated experiments. Although thereason is unclear why an ohmic contact is obtained by using the abovementioned metals or alloys, some possibilities can be considered.

IVa-family elements

When a IVa-family metal such as Ti, Zr, Hf, TiC, ZrNi, or the like, oran alloy containing a IVa-family metal is formed as an electrode on ann-type cubic boron nitride, the electrode constituent element/elementsas well as boron and nitrogen are activated by the heating of asubstrate at the time of electrode formation, radiation heat at the timeof electrode evaporation, annealing after the electrode formation, etc.,so that mutual diffusion is generated in an interface between theelectrode and the semiconductor.

The IVa-family elements (Ti, Zr, and Hf), on the other hand, are alsoapt to react with boron and nitrogen so as to easily form a boride suchas TiB₂, ZrB₂, HfB₂, etc. and a nitride such as TiN, ZrN, HfN, etc. Thisphenomenon is well known. Therefore, the mutual diffusion in theelectrode constituent elements as well as boron and nitrogen is furtheraccelerated. Moreover, a metal compound layer is formed in theelectrode/semiconductor interface in which the mutual diffusion isgenerated, and the substantial electrode/semiconductor interface isshifted into a semiconductor layer so as to form a clean interface.

Further, there is a possibility that the IVa-family elements (Ti, Zr,and Hf) diffused into the cubic boron nitride become donors for thecubic boron nitride. There is a further possibility that holes ordefects formed at places where boron and nitrogen have been removed bythe diffusion become donors. As a result, an n-layer (n⁺ layer) dopedwith high concentration is formed in the region where the mutualdiffusion has been generated. Although the cubic boron nitride is of then-type originally, it becomes of the n-type having a dopingconcentration considerably higher than the original mean dopingconcentration. The thickness of the barrier layer is reduced by thehigh-concentration doping so that the contact becomes ohmic by atunneling current.

Si or S

Similarly, when a metal or alloy containing Si or S is formed as anelectrode on an n-type cubic boron nitride, the Si or S contained in theelectrode is activated by heating of a substrate at the time ofelectrode formation, radiation heat at the time of electrodeevaporation, annealing after the electrode formation, etc., so that theSi or S gradually diffuses into the adjacent cubic boron nitride.

The Si or S acts as an effective donor in the cubic boron nitride. Thisphenomenon is well known. Therefore, in the region where Si or S hasbeen diffused, an n-layer (n⁺ -layer) doped with high concentration isformed.

B, Al, Ga, or In

Similarly when a metal or alloy consisting of at least one of B, Al, Gaand In is formed as an electrode on an n-type cubic boron nitride, anelectrode constituent element/elements as well as boron and nitrogen areactivated by heating of a substrate at the time of electrode formation,radiation heat at the time of electrode evaporation, annealing after theelectrode formation, etc., so that mutual diffusion is generated in aninterface between the electrode and the semiconductor.

Such elements as Al, Ga, In, etc. easily react with nitrogen so as toeasily form nitride such as AlN, GaN, InN, etc. This phenomenon is wellknown. Also in the case where an electrode includes B, of course,nitrogen diffuses into B. Further, Al easily forms boride such as AlB₂,etc. This is well known. Therefore, the mutual diffusion in theelectrode constituent elements as well as boron and nitrogen is furtheraccelerated. Moreover, a metal compound layer is formed in theelectrode/semiconductor interface in which the mutual diffusion isgenerated, and the substantial electrode/semiconductor interface isshifted into a semiconductor layer so as to form a clean interface.

Va-family metal

Similarly, when a Va-family metal such as V, Nb, Ta, VC, Nb₃ Al, or thelike, or an alloy containing a Va-family metal is formed as an electrodeon an n-type cubic boron nitride, an electrode constituentelement/elements as well as boron and nitrogen are activated by heatingof a substrate at the time of electrode formation, radiation heat at thetime of electrode evaporation, annealing after the electrode formation,etc., so that mutual diffusion is generated in an interface between theelectrode and the semiconductor.

The Va-family elements (V, Nb, and Ta) are also apt to react with boronand nitrogen so as to easily form a boride such as VB₂, NbB₂, TaB₂, etc.and a nitride such as VN, NbN, TaN, etc. This phenomenon is well known.Therefore, the mutual diffusion in the electrode constituent elements aswell as B and N is further accelerated. Moreover, a metal compound layeris formed in the electrode/semiconductor interface in which the mutualdiffusion is generated, and the substantial electrode/semiconductorinterface is shifted into a semiconductor layer so as to form a cleaninterface.

Presently for all four types of coatings, there is the possibility thatholes or defects formed at places where boron or nitrogen have beenremoved by the diffusion thereof into the electrode act as donors. Infact, in the case of GaN which is a III-V family compound and which isof the same kind as boron nitride, extreme n-type semiconductorcharacteristics are shown without any doping. This is, perhaps, becauseholes at which nitrogen has been removed act as donors.

As a result of mutual diffusion, an n-layer (n⁺ -layer) in which highconcentration donor is formed in the region in the vicinity of theinterface between the electrode and the cubic boron nitride. Althoughthe cubic boron nitride is of the n-type originally, it becomes of then-type having a donor concentration considerably higher than theoriginal mean donor concentration. The thickness of the barrier layer isreduced by the high-concentration donor so that the contact becomesohmic by a tunneling current.

In an ideal case, whether a metal/semiconductor contact becomes an ohmicone or a Schottky one is determined depending on the work function(φ_(m)) of the metal and the electron affinity (X_(s)) of thesemiconductor. Here, the work function of a metal is energy required fordrawing out electrons in Fermi-level to an infinite distance. Theelectron affinity of a semiconductor is energy required for drawing outelectrons existing in the bottom of a conduction band to an infinitedistance. In the case of an n-type semiconductor, the height (φ_(B)) ofa barrier in a metal/semiconductor contact is expressed by the followingequation.

    φ.sub.B =φ.sub.m -X.sub.s                          (1)

The contact becomes therefore:

an ohmic one when φ_(B) <0; and

a Schottky one when φ_(B) >0.

Since φ_(B) represents the height of the barrier, electrons are checkedby the barrier if the value of φ_(B) is positive so that the contactbecomes a Schottky one, while if the value of φ_(B) is negative, thereis no barrier so that the contact becomes an ohmic one. It seems, fromthe above equation, that an ohmic contact can be obtained if a metalhaving a low work function is selected to be an electrode.

In an actual metal/semiconductor contact, however, defects exist in ametal/semiconductor contact interface so as to form localized states(also called interface states, φ₀ : interface level) in the vicinity ofthe center in a band gap. If a large number of defects exist, theFermi-level of the semiconductor is pinned up to the level of φ₀. As aresult, if the band gap of the semiconductor is represented by E_(g),φ_(B) is expressed by the following equation, and hardly depends on thekind of metal at all.

    φ.sub.B =E.sub.s -φ.sub.0                          (2)

The φ is measured from the upper end of the valence band. This isbecause electrons from the metal can freely flow into the localizedstates to occupy the states so that the upper end of the localizedstates coincides with the Fermi plane of the metal and the Fermi levelof the semiconductor coincides with the Fermi plane of the metal.

Since it is the case, of course, that φ_(B) >0, the metal/semiconductorcontact becomes a Schottky one. This phenomenon is known as the pinningeffect, and is remarkable particularly in the case of III-V familycompound semiconductors. Further, φ₀ is about 1/3-1/2 times as large asE_(g).

Also in the case of a cubic boron nitride, various kinds of interfacedefects may exist because it is not a single element material. Further,it is thought that a large number of interface defects exist whichthereby cause the pinning effect because a cubic boron nitride crystalin the present circumstances is imperfect and has a large number ofdefects.

FIG. 1 is a band diagram of a Schottky contact between a metal and ann-type semiconductor. A Fermi-level 2 of the semiconductor is continuedfrom a Fermi-level 1 of the outside metal. A conduction band and avalence band exist in the semiconductor. The reference numerals 3 and 4designate the lower end of the conduction band and the upper end of thebalance band respectively. Since the semiconductor is of the n-type, thelower end 3 of the conduction band is approximate to the Fermi-level 2.The Fermi-level is pinned up to the interface level φ₀ at a boundary 5,so that a barrier having a height of φ_(B) is formed. The band isextremely bent upward in the vicinity of the boundary 5 because thedonor density is high at the boundary. Although electrons accumulate ina downward curved portion 6 in the conduction band, the electrons mayfly into the metal because of the tunneling effect. Since all the donorsat the metal/semiconductor boundary are donated electrons and havepositive charges of e. The positive charges are substantially uniformlydistributed in the vicinity of the boundary and therefore this portionbecomes a depletion layer in which no electron exists. The thickness ofthis portion can be calculated as follows. The following equation isestablished by integrating the Gaussian expression relating to anelectric field and electric charges.

    d=[2·ε.sub.S ·ε.sub.0 ·(φ.sub.B -V)/(e·N.sub.d)].sup.1/2  (3)

d: depletion layer width

ε_(S) : specific inductive capacity of the semiconductor

ε₀ : dielectric constant in the vacuum

V: applied voltage

e: elementary electric charge

N_(d) : donor density in the semiconductor

Although φ_(B) is a potential having an energy dimension as before,φ_(B) is treated below as a value of voltage obtained by dividing thepotential by the elementary electric charge e.

As seen from the equation (3), the depletion layer width d becomessmaller as the donor density in the semiconductor becomes higher. Sinceelectrons easily pass through the thin depletion layer, the contactcomes to have low resistance. This phenomenon will be described in moredetail.

Being a tunneling current, a current I per unit area can be expressed asfollows. ##EQU1## in which U represents the potential of a conductionelectron measured from the bottom of the conduction band.

U is expressed as follows as a function of x by integrating the Gaussianexpression.

    U=e(φ.sub.B -V){1-(x/d)}.sup.2                         (5)

    A=4πem.sub.e k.sup.2 /h.sup.3                           (6)

in which, me represents the effective mass of electrons in thesemiconductor, h represents the Planck's constant, k represents theBoltzmann constant, and T represents the absolute temperature. In thecase of free electrons, A is 120 A/cm² K². If integration is made withrespect to x, the following expression is obtained.

    I=AT.sup.2 exp{-(φ.sub.B -V)/V.sub.00 }                (7)

in which

    V.sub.00 =hNd.sup.1/2 /4π(ε.sub.0 ε.sub.S m.sub.e).sup.1/2                                          (8)

Assuming that Nd=10²¹ cm⁻³, m_(e) represents the free electron mass, andεhd S=1, ε₀ =1, and T=300° K., then V₀₀ is 0.56 V. Although thedefinition of φ_(B) has been described above, it is considered that ifan n-type impurity is doped with high concentration, the φ_(B) itselfdecreases. This is because, as the localized states increase, φ₀increases. E_(g) is 7 eV, while φ_(B) decreases to about 6-3 V. It isassumed that the concentration of doping is extremely large in thevicinity of the boundary. If V₀₀ =0.5 V, this contact can be regardedsubstantially as an ohmic one from the equation (6). This is the caseprovided that the direction of the voltage from the semiconductor sidetoward the metal electrode is defined to be positive.

Resistance (R_(C)) is obtained by partially differentiating the currentwith respect to the voltage on the assumption that V=0.

The R_(C) at this time is expressed by the following equation.

    R.sub.C =(V.sub.00 /AT.sup.2)exp(φ.sub.B /V.sub.00)    (9)

Assuming that Nd=10²¹ cm⁻³, the mass of free electrons is represented bym_(e), εo=1, and T=300° K., then V₀₀ =0.56 V. If the coefficient beforeexp. is included in exp., the following equation is obtained.

    R.sub.C =exp{(φ.sub.B /0.56)-16.77)                    (10)

If φ_(B) =6 V, Rc becomes 10⁻³ Ω.cm², while if φ_(B) =7 V, Rc becomes10⁻² Ω.cm².

Assuming that Nd=10²⁰ cm⁻³, then V00 becomes 0.18 V and the followingequation is obtained.

    R.sub.C =exp{(φ.sub.B /0.18)-17.91}                    (11)

if φ_(B) =3 V, R_(c) is about 10⁻¹ Ω.cm², while if φ_(B) =2.5 V, Rc isabout 10⁻² Ω.cm².

Thus, if an n-type impurity is doped with high concentration, thedepletion layer thickness d decreases. Further, there is such an effectthat the localized state of the surface is increased by highconcentration doping so that φ_(O) increases and φ_(B) decreases.Although description has been made as to the case where φ_(B) is about1/2-2/3 times as large as the band gap (7 eV), it is considered that theφ_(B) may be further reduced.

Thus, the higher the donor density is made by high concentration doping,the more the thickness of the depletion layer decreases and the moreφ_(B) decreases so as to make a tunneling current flow easily. Thus, itcan be expected that the contact becomes ohmic. But the fact is not yetclearly understood. It may be that the localized state is transmitted bythe tunnelling of electrons. Although such an effect can be expected bydoping an impurity with high concentration, if the density of theimpurity becomes so high, the result is rather undesirable because theresistance of the electrode increases. It is therefore believed thatthere exists a suitable range of the concentration.

Further, the following possibility is considered. That is, a boridelayer and a nitride layer have been formed in a metal/semiconductorinterface so that a large number of defects have been brought into theinterface and the vicinity of the interface. If the localized states 7of high density exist in the vicinity of the boundary 5 as shown in FIG.2, electrons may move between the metal and the semiconductor throughthe states so that a current may flow between them. Since the higher thelocalized state density is, the more easily electrons move, which makesthe contact resistance become low. Moreover, since the movement isperformed bi-directionally, the contact becomes ohmic.

Other than those mechanisms described above, various mechanisms forobtaining an ohmic contact can be considered. They cannot be specifiedat the present time. An ohmic contact, however, can be obtained on ann-type cubic boron by forming an electrode according to the presentinvention.

EXAMPLES

The ohmic contact electrode according to the present invention will bedescribed more in detail with reference to examples.

In order to inspect the kind of electrode material to which theelectrode structure of the present invention was suitable, electrodeswere produced from various materials and the ohmic property and contactresistance were evaluated.

Example 1

Ti, TiAl, TiB₂, TiC, TiCr, TiFe, TiN, TiNb, TiNi, TiSi, Zr, ZrAl, ZrB₂,ZrC, ZrN, ZrNb, ZrNi, ZrS₂, ZrSi₂, Hf, HfB₂, HfC, HfN, HfS₂, and HfSi₂were formed, as the electrodes, according to the present invention, on ahigh-pressure synthesized Si-doped n-type cubic boron nitride, and theohmic property and contact resistance of the thus formed electrodes wereevaluated. The electrodes were formed at a substrate temperature of 400°C. by vacuum evaporation or sputtering depending on the material.

At that time, an electrode pattern shown in FIG. 3 was formed on thecubic boron nitride by using a metal mask. The thickness of each of fourelectrodes was about 0.2-0.3 μm. The length was 0.8 mm and the width was0.1 mm. They were arranged in parallel to each other at intervals of 0.1mm, 0.2 mm, and 0.3 mm.

The current-voltage characteristic between arbitrary electrodes wasmeasured in a voltage range of from -10 V to +10 V to thereby Judge theohmic property of the electrodes. With respect to the electrodes ofwhich the ohmic characteristic has been measured, the contact resistancewas measured by using a transmission line model on the basis of theresistance and electrode distance between two electrodes of theelectrode pattern of FIG. 3.

Table 1 shows the summary of the results of applying one of the abovemetals to form an ohmic contact electrode.

                  TABLE 1                                                         ______________________________________                                        Electrodes Containing IVa-Family Metal, Current-                              Voltage Characteristic, and Contact Resistance                                                                  Contact                                     Electrode Method of   Current-voltage                                                                           Resistance                                  (atom ratio)                                                                            Formation   Characteristics                                                                           (Ω · cm.sup.2)               ______________________________________                                        Ti        Vacuum evap.                                                                              Ohmic       2 × 10.sup.-1                         TiA1      Sputtering  Ohmic       3 × 10.sup.-1                         TiB.sub.2 Sputtering  Ohmic       8 × 10.sup.-2                         TiC       Sputtering  Ohmic       1 × 10.sup.-1                         TiCr(80:20)                                                                             Sputtering  Ohmic       5 × 10.sup.-1                         TiFe      Sputtering  Ohmic       4 × 10.sup.-1                         TiN       Sputtering  Ohmic       7 × 10.sup.-2                         TiNb(66:34)                                                                             Sputtering  Ohmic       3 × 10.sup.-1                         TiNi(55:45)                                                                             Sputtering  Ohmic       7 × 10.sup.-1                         TiSi.sub.2                                                                              Sputtering  Ohmic       4 × 10.sup.-2                         Zr        Vacuum evap.                                                                              Ohmic       1 × 10.sup.-1                         ZrAl(75:25)                                                                             Sputtering  Ohmic       3 × 10.sup.-1                         ZrB.sub.2 Sputtering  Ohmic       7 × 10-2                              ZrC       Sputtering  Ohmic       9 × 10.sup.-2                         ZrN       Sputtering  Ohmic       6 × 10.sup.-2                         ZrNb(95:5)                                                                              Sputtering  Ohmic       5 × 10.sup.-1                         ZrNi      Sputtering  Ohmic       6 × 10.sup. -1                        ZrS.sub.2 Sputtering  Ohmic       4 × 10.sup.-2                         ZrSi.sub.2                                                                              Sputtering  Ohmic       5 × 10.sup.-2                         Hf        Vacuum evap.                                                                              Ohmic       8 × 10.sup.-2                         HfB.sub.2 Sputtering  Ohmic       6 × 10.sup.-2                         HfC       Sputtering  Ohmic       2 × 10.sup.-1                         HfN       Sputtering  Ohmic       9 × 10.sup.-2                         HfS.sub.2 Sputtering  Ohmic       6 × 10.sup.-2                         HfSi.sub.2                                                                              Sputtering  Ohmic       4 × 10.sup.-2                         ______________________________________                                    

As seen in Table 1, IVa-family metals such as Ti, Zr, Hf, TiC, ZrNi,etc. and alloys containing a IVa-family metal were formed, as theelectrodes, on an n-type cubic boron nitride, so that the ohmiccharacteristic could be obtained although the contact resistance was avalue within a range from 10⁻² to 10-1 Ω.cm².

Example 2

Si or S was added to Cr, W, Mn, Co, Ni, Pt, Ag, Zn, WC, MnZn, Ni₂ B,PtPd, or AgAl. As the cubic boron nitride, a high-pressure compositionwas used. The electrodes were formed at a substrate temperature of 400°C., and by the use of vacuum evaporation or sputtering depending on thematerial. As the electrode materials, used were those in which each ofthe Si-concentration and S-concentration was 0.1%-0.5%.

At that time, an electrode pattern shown in FIG. 3 was formed on thecubic boron nitride by using a metal mask. The thickness of each of fourelectrodes was about 0.5-0.3 μm. The length was 0.8 mm and the width was0.1 mm. They were arranged in parallel to each other at intervals of 0.1mm, 0.2 mm, and 0.3 mm. The current-voltage characteristic betweenarbitrary electrodes was measured in a voltage range of from -10 V to+10 V to thereby judge the ohmic property of the electrodes. Withrespect to the electrodes the ohmic characteristic of which has beenmeasured, the contact resistance was measured by using a transmissionline model on the basis of the resistance and electrode distance betweentwo electrodes of the electrode pattern of FIG. 3.

The results of the above examples in which either Si or S were addedwith the above metals to form electrodes are summarized in Tables 2 and3.

                  TABLE 2                                                         ______________________________________                                        Electrodes Containing Si, Current-                                            Voltage Characteristic, and Contact Resistance                                                                  Contact                                              Method of   Current-Voltage                                                                            Resistance                                  Electrode                                                                              Formation   Characteristic                                                                             (Ω · cm.sup.2)               ______________________________________                                        CrSi     Vacuum evap.                                                                              Ohmic        1 × 10.sup.-1                         WSi      Vacuum evap.                                                                              Ohmic        4 × 10.sup.-2                         MnSi     Vacuum evap.                                                                              Ohmic        2 × 10.sup.-1                         CoSi     Vacuum evap.                                                                              Ohmic        6 × 10.sup.-2                         NiSi     Vacuum evap.                                                                              Ohmic        6 × 10.sup.-2                         PtSi     Vacuum evap.                                                                              Ohmic        7 × 10.sup.-2                         AgSi     Vacuum evap.                                                                              Ohmic        8 × 10.sup.-2                         ZnSi     Vacuum evap.                                                                              Ohmic        5 × 10.sup.-2                         WCSi     Sputtering  Ohmic        2 × 10.sup.-2                         MnZnSi   Sputtering  Ohmic        9 × 10.sup.-2                         Ni.sub.2 BSi                                                                           Sputtering  Ohmic        8 × 10.sup.-2                         PtPdSi   Sputtering  Ohmic        2 × 10.sup.-1                         AgAlSi   Sputtering  Ohmic        9 × 10.sup.-2                         ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Electrodes Containing S, Current-                                             Voltage Characteristic, and Contact Resistance                                                                  Contact                                              Method of   Current-Voltage                                                                            Resistance                                  Electrode                                                                              Formation   Characteristic                                                                             (Ω · cm.sup.2)               ______________________________________                                        CrS      Vacuum evap.                                                                              Ohmic        2 × 10.sup.-1                         WS       Vacuum evap.                                                                              Ohmic        2 × 10.sup.-2                         MnS      Vacuum evap.                                                                              Ohmic        9 × 10.sup.-2                         CoS      Vacuum evap.                                                                              Ohmic        7 × 10.sup.-2                         NiS      Vacuum evap.                                                                              Ohmic        1 × 10.sup.-1                         PtS      Vacuum evap.                                                                              Ohmic        8 × 10.sup.-2                         AgS      Vacuum evap.                                                                              Ohmic        6 × 10.sup.-2                         ZnS      Vacuum evap.                                                                              Ohmic        8 × 10.sup.-2                         WCS      Sputtering  Ohmic        2 × 10.sup.-2                         MnZnS    Sputtering  Ohmic        7 × 10.sup.-2                         Ni.sub.2 BS                                                                            Sputtering  Ohmic        6 × 10.sup.-2                         PtPdS    Sputtering  Ohmic        1 × 10.sup.-1                         AgAlS    Sputtering  Ohmic        8 × 10.sup.-1                         ______________________________________                                         (Here, MS does not mean that the atom ratio of a metal or an alloy M to S     is 1:1 but means that a certain quantity of S is contained in the metal o     alloy M. That is, MS does not mean a molecular formula with respect to S.                                                                              

Inspection was made on various metal materials and metal compounds, andas seen in Tables 2 and 3, the ohmic characteristic could be obtainedwith respect to all the material, although the contact resistance was avalue within a range from 10⁻² to 10⁻¹ Ω.cm².

This fact makes reasonable the aforementioned estimation of the electrontunnel effect due to high concentration donor generation at the boundarybetween a metal and a semiconductor.

Example 3

When a metal or a metal compound containing Si or S is used as amaterial for the electrode, inspection was made to obtain a properconcentration of Si or S.

The electrodes were made from PtSi or PtS (this does not mean that theatomic ratio is 1:1 but means that a certain quantity of Si or S iscontained in Pt) under the condition that the respective concentrationsof Si and S were changed variously. The results of evaluation of theohmic characteristic and contact resistance are shown in Tables 4 and 5.

As the cubic boron nitride, a high-pressure composition was used. Theelectrodes were formed through a vacuum evaporation method withoutheating the substrates and subjected to annealing in an atmosphere of N₂+H₂ (H₂ :10%) at 800° C. for 10 minutes.

                  TABLE 4                                                         ______________________________________                                        Si Concentration, Current-Voltage                                             Characteristic, and Contact Resistance                                        Si                         Contact                                            Concentration  Current-Voltage                                                                           Resistance                                         (PtSi)         Characteristic                                                                            (Ω · cm.sup.2)                      ______________________________________                                        0.01%          Ohmic       2 × 10.sup.0                                 0.1%           Ohmic       1 × 10.sup.-1                                1.5%           Ohmic       6 × 10.sup.-2                                1.0%           Ohmic       5 × 10.sup.-2                                5.0%           Ohmic       2 × 10.sup.-1                                10.0%          Ohmic       3 × 10.sup.-1                                ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        S Concentration, Current-Voltage                                              Characteristic, and Contact Resistance                                                                   Contact                                            S Concentration                                                                              Current-Voltage                                                                           Resistance                                         (PtS)          Characteristic                                                                            (Ω · cm.sup.2)                      ______________________________________                                        0.01%          Ohmic       1 × 10.sup.0                                 0.1%           Ohmic       9 × 10.sup.-2                                0.5%           Ohmic       4 × 10.sup.-2                                1.0%           Ohmic       4 × 10.sup.-2                                5.0%           Ohmic       1 × 10.sup.-1                                10.0%          Ohmic       2 × 10.sup.-1                                ______________________________________                                    

From the Tables 4 and 5, it can be seen that the ohmic contact could beobtained over all the range of Si- or S-concentration, and that althoughthe contact resistance decreased as the Si- or S-concentrationincreases, it reached the minimum at several % and then increasedthereafter. Further, it is not desirable to make the Si- orS-concentration excessively high, because the resistance of theelectrode per se becomes high. In practice, it suffices to select theSi- or S-concentration to be about 1% by weight.

Example 4

B, AlB₂, CrB₂, SiB₄, WB, Al, AlCu, AlSi, Ga, GaIn, GaSn, GaZn, In, InMg,and InSn were formed, as the electrodes, according to the presentinvention, on a high-pressure synthesized Si-doped n-type cubic boronnitride, and the ohmic property of the thus formed electrodes wasevaluated. In the case of low melting point metals and alloys such asGa, In, GaIn, etc., after electrode materials were set at two positionson a cubic boron nitride, electrodes were formed through alloyingtreatment in an N₂ atmosphere at 400° C. for five minutes. Thecurrent-voltage characteristic between those electrodes was measured ina voltage range of from -10 V to +10 V to thereby judge the ohmicproperty of the electrodes.

The other electrodes were formed by using a vacuum evaporation method ora sputtering method depending on the materials under the condition thatthe substrate temperature was kept at room temperature. After formationof the electrodes, annealing was made in an N₂ atmosphere at 400° C. for20 minutes. Further, in formation of the electrodes, an electrodepattern shown in FIG. 3 was formed on the cubic boron nitride by using ametal mask. The thickness of each of four electrodes was about 0.2-0.3μm. The length was 0.8 mm and the width was 0.1 mm. They were arrangedin parallel to each other at intervals of 0.1 mm, 0.2 mm, and 0.3 mm.With respect to the electrodes the ohmic characteristic of which hasbeen measured, the contact resistance was measured by using atransmission line model on the basis of the resistance and electrodedistance between two electrodes of the electrode pattern of FIG. 3.

Table 6 shows the summary of the results of the above examples.

                  TABLE 6                                                         ______________________________________                                        Electrodes Containing B, Al, Ga, or In; Current-                              Voltage Characteristic and Contact Resistance                                                                   Contact                                     Electrode                                                                              Method of   Current-Voltage                                                                            Resistance                                  (atom ratio)                                                                           Formation   Characteristic                                                                             (Ω · cm.sup.2)               ______________________________________                                        B        Vacuum evap.                                                                              Ohmic        4 × 10.sup.-1                         AlB.sub.2                                                                              Sputtering  Ohmic        9 × 10.sup.-2                         CrB.sub.2                                                                              Sputtering  Ohmic        4 × 10.sup.-1                         SiB.sub.4                                                                              Sputtering  Ohmic        5 × 10.sup.-2                         WB       Sputtering  Ohmic        3 × 10.sup.-1                         Al       Vacuum evap.                                                                              Ohmic        2 × 10.sup.-1                         AlCu     Sputtering  Ohmic        5 × 10.sup.-1                         AlSi     Sputtering  Ohmic        8 × 10.sup.-2                         Ga       Alloying    Ohmic        --                                          GaIn     Alloying    Ohmic        --                                          GaSn     Alloying    Ohmic        --                                          GaZn     Sputtering  Ohmic        6 × 10.sup.-1                         In       Alloying    Ohmic        --                                          InMg     Sputtering  Ohmic        7 × 10.sup.-1                         InSn     Alloying    Ohmic        --                                          ______________________________________                                    

As seen in Table 6, a metal or alloy consisting of at least one of theelements B, Al, Ga and In was formed, as an electrode, on an n-typecubic boron nitride, so that the ohmic characteristic could be obtained.The contact resistance of the electrode formed through vacuumevaporation or sputtering was within an order of 10⁻² --10⁻¹ Ω.cm².

Example 5

V, VB₂, VC, V₃ Co, VaG₃, VN, VS, V₃ Si, V₂ Zr, Nb, Nb₃ Al, NbB₂, NbC,Nb₃ Ga, Nb₃ Ge, NbN, NbS₂, NbSi₂, Ta, TAB₂, TaC, TaN, TaS₂, and TaSi₂were formed, as the electrodes according to the present invention, on ahigh-pressure synthesized Si-doped n-type cubic boron nitride, and theohmic property and contact resistance of the thus formed electrodes wereevaluated. The electrodes were formed at a substrate temperature of 400°C. by vacuum evaporation or sputtering depending on the material.

At that time, an electrode pattern shown in FIG. 3 was formed on thecubic boron nitride by using a metal mask. The thickness of each of fourelectrodes was about 0.2-0.3 μm. The length was 0.8 mm and the width was0.1 mm. They were arranged in parallel to each other at intervals of 0.1mm, 0.2 mm, and 0.3 mm. The current-voltage characteristic betweenarbitrary electrodes was measured in a voltage range of from -10 V to+10 V to thereby judge the ohmic property of the electrodes. Withrespect to the electrodes the ohmic characteristic of which has beenmeasured, the contact resistance was measured by using a transmissionline model on the basis of the resistance and electrode distance betweentwo electrodes of the electrode pattern of FIG. 3.

Table 7 shows the summary of the results of the above example.

                  TABLE 7                                                         ______________________________________                                        Electrodes Containing IVa-Family Metal, Current-                              Voltage Characteristic, and Contact Resistance                                                                  Contact                                     Electrode                                                                              Method of   Current-Voltage                                                                            Resistance                                  (atom ratio)                                                                           Formation   Characteristic                                                                             (Ω · cm.sup.2)               ______________________________________                                        V        Vacuum evap.                                                                              Ohmic        3 × 10.sup.-1                         VB.sub.2 Sputtering  Ohmic        9 × 10.sup.-2                         VC       Sputtering  Ohmic        1 × 10.sup.-1                         V.sub.3 Co                                                                             Sputtering  Ohmic        7 × 10.sup.-1                         V.sub.3 Ga                                                                             Sputtering  Ohmic        5 × 10.sup.-1                         VN       Sputtering  Ohmic        8 × 10.sup.-2                         VS       Sputtering  Ohmic        5 × 10.sup.-2                         V.sub.3 Si                                                                             Sputtering  Ohmic        6 × 10.sup.-2                         V.sub.2 Zr                                                                             Sputtering  Ohmic        2 × 10.sup.-1                         Nb       Vacuum evap.                                                                              Ohmic        3 × 10.sup.-1                         Nb.sub.3 Al                                                                            Sputtering  Ohmic        5 × 10.sup.-1                         NbB.sub.2                                                                              Sputtering  Ohmic        8 × 10.sup.-2                         NbC      Sputtering  Ohmic        1 × 10.sup.-1                         Nb.sub.3 Ga                                                                            Sputtering  Ohmic        5 × 10.sup.-1                         Nb.sub.3 Ge                                                                            Sputtering  Ohmic        6 × 10.sup.-1                         NbN      Sputtering  Ohmic        9 × 10.sup.-2                         NbS.sub.2                                                                              Sputtering  Ohmic        5 × 10.sup.-2                         NbSi.sub.2                                                                             Sputtering  Ohmic        4 × 10.sup.-2                         Ta       Vacuum evap.                                                                              Ohmic        1 × 10.sup.-1                         TaB.sub.2                                                                              Sputtering  Ohmic        8 × 10.sup.-2                         TaC      Sputtering  Ohmic        1 × 10.sup.-1                         TaN      Sputtering  Ohmic        9 × 10.sup.-2                         TaS.sub.2                                                                              Sputtering  Ohmic        3 × 10.sup.-2                         TaSi.sub.2                                                                             Sputtering  Ohmic        4 × 10.sup.-2                         ______________________________________                                    

As seen in Table 7, Va-family metals such as V, Nb, Ta, VC, Nb₃ Al, etc.and alloys containing a Va-family metal were formed, as the electrodes,on an n-type cubic boron nitride, so that the ohmic characteristic couldbe obtained although the contact resistance was a value within a rangefrom 10⁻² to 10⁻ Ω.cm².

It is effective to perform suitable annealing after formation ofelectrodes with the metals used in the above examples 1-5. Although theoptimum values of the annealing temperature and time vary depending onthe electrode material to be used, it is preferable to select theannealing temperature to be about 300° C.-1800° C.

Since oxidation of the electrode may progress at the time of annealingdepending on the electrode material, it is desirable to perform theannealing in an inert gas, N₂ or H₂, or in a vacuum. Further, in thecase where the temperature is as high as 1000° C. or more, it ispreferable to perform annealing in an inert gas, N₂ or H₂, or in avacuum, similarly to the above case, because of the oxidization orhexagonal boron-nitriding of the cubic boron nitride progresses when O₂is present.

Moreover, it is more effective to use a high temperature at the time offormation of the electrodes in place of annealing. It is desirable toselect this temperature to be 300° C.-1800° C., although it depends onthe electrode material. It is also more effective that a metal alloy ofknown dopant (Si, S, or the like) and a IVa-family element (Ti, Zr, orHf) is formed, as the electrodes, on the n-type semiconductor cubicboron nitride. Moreover, if the electrodes according to the presentinvention and metal electrodes coated with Au, Ag, or the like havinglow resistance, are laminated to be a multilayer structure, the effectbecomes more remarkable.

As described above, a IVa-family metal (Ti, Zr, or Hf) or an alloycontaining a IVa-family metal; a metal or an alloy containing Si or S; ametal or alloy consisting of at least one of B, Al, Ga and In; or aVa-family metal or an alloy containing a Va-family metal is formed, asan electrode, on an n-type semiconductor cubic boron nitride, so that anohmic contact can be obtained with respect to the n-type semiconductorboron nitride.

Since this technique is indispensable to produce semiconductor cubicboron nitride devices, it is effective in formation of all kinds ofdevices.

What is claimed is:
 1. An ohmic contact electrode for an n-typesemiconductor cubic boron nitride comprising at least one layer of amaterial selected from the group consisting of a IVa metal and an alloywith a IVa metal.
 2. An ohmic contact electrode for an n-typesemiconductor cubic boron nitride comprising at least one layer of amaterial selected from the group consisting of Ti, TiAl, TiB₂, TiC,TiCr, TiFe, TiN, TiNb, TiNi, TiSi, Zr, ZrAl, ZrB₂, ZrC, ZrN, ZrNb, ZrNi,ZrS₂, ZrSi₂, Hf, HfB₂, HfC, HfN, HfS₂, and HfSi₂.
 3. An ohmic contactelectrode for an n-type semiconductor cubic boron nitride comprising atleast one layer of a metal selected from the group consisting of Au andAg.